Fish muscle comprises fibres having a typical diameter of 30 to 200 microns. These fibres comprise myofibrils having a typical diameter of 1 to 2 microns.
The myofibril is built up of many sarcomeres aligned end to end. One sarcomere may be defined as the structure lying between adjacent lines AA and BB of FIG. 1.
Within each sarcomere are thin filaments 2, comprising mainly the protein actin, together with thick filaments 4, comprising mainly the protein myosin. These filaments are organised into a lattice structure, as illustrated by the cross sectional views of FIG. 1.
The myosin molecule consists of a head region 6 and a tail region 8.
It has been shown by Yamamoto et al, Biosci Biotech Biochem, 57, 383 (1993) that the application of ultra high pressure will denature the head region of the myosin molecule and that this denaturation is likely to occur at lower pressures than those required to denature the myosin tail region. It has also been shown that pressures below 500 MPa do not fully denature the myosin molecule (McArthur & Wilding, Gums and Stabilisers in the Food Industry Vol 8, Phillips, Williams & Wedlock, IRL Press, 309-317 (1995)).
Fish, either as fillets or mince or as part of a more complex product such as fish fingers, has the problem that it deteriorates on prolonged frozen storage (for example, storage for 6 months at -10.degree. C.). On thawing, the texture of the fish is quite different to the texture of fresh fish. Typically, the thawed fish appears wet, will lose water even on gentle squeezing, and has an opaque appearance. The differences between fresh fish and thawed fish persist after cooking; the thawed fish has a grey rather than white appearance, is more chewy, dry and fibrous, and exhibits increased water loss during cooking.
The rate of textural decline of frozen fish is predominantly governed by the temperature at which it is stored and the fish species: fish of the Gadoid species (eg Cod, Alaska Pollack, Saithe, Whiting and Haddock) are particularly prone to frozen deterioration.
The textural changes resulting from the freezing and frozen storage of fish have been attributed to changes in the myofibrils.
Jarenback et al, in the Journal of Food Technology, vol 10, p 229 (1975), observed a decrease in the dimensions of the lattice structure described above in frozen cod samples which had undergone textural deterioration. Also, they observed disturbances to the lattice.
It is understood that during freezing and frozen storage, water is displaced from myofibrils and forms ice crystals. Upon thawing, the water is not able to return to its original location because the myofibrils' lattice structure is not able to return to its original dimensions. Hence, textural deterioration results.
It is therefore desirable to be able to slow or prevent frozen deterioration, so enabling displaced water to return to myofibrils upon thawing.
We have now shown that it is possible to provide fish with at least some resistance to frozen deterioration, by subjecting the fish to a treatment which irreversibly changes the conformation of the head region of the myosin molecule, such that it is unable to revert back to the conformation observed in untreated samples, but does not completely and irreversibly change the conformation of the actin molecule. This means that the myosin head region is at least partially denatured as a result of this treatment, whilst the actin molecule may be partially denatured but is not completely denatured.
An article by D Farr in Trends in Food Science & Technology, vol 1, 1990, p 14-16 discusses the effects of high hydrostatic pressure on protein denaturation and states that such denaturation has been attributed to the pressure-induced unfolding of the protein chains. It also mentions that the use of moderate hydrostatic pressures, in combination with sub-zero temperatures, has been proposed as a means of storing food products without the formation of ice, thereby avoiding damage due to freezing.
This avoidance of damage is achieved by applying pressure to lower the freezing point of water and storing the food product at a sub-zero temperature which is greater than the lowered freezing point of water, such that the water in the food product does not freeze and thus can not form damaging ice crystals.
An article by D E Johnston in Chemistry & Industry, no 13, 1994, p 499-501 mentions that pressure treatment can result in reversible or irreversible enzyme denaturation, and that high pressure offers a means of controlled protein unfolding. It also states that rapid uniform freezing can be made to take place by cooling a food below its freezing point under high pressure and then releasing the pressure, thus giving rise to less textural damage when the food is thawed.
As a result of the almost immediate freezing provided by this technique, ice crystals have little time to grow so they are very small and distributed evenly throughout the food. Hence, textural damage is reduced in comparison to a traditionally frozen food, which contains bigger ice crystals, which cause greater textural damage as a consequence of their size.
However, a problem with this technique is that heat is generated as the ice is formed. If this heat is not removed, the food rises in temperature and thaws. Another problem is that the technique does not prevent the small ice crystals from growing during frozen storage, so textural damage may still occur.
An article by I N A Ashie and B K Simpson in Food Research International, Vol 29 No5-6, pp569-575, 1996, discusses the effect of high hydrostatic pressure on seafood enzymes and relates this to seafood texture deterioration. However, there is no discussion of the effect of pressure on myosin and actin molecules in fish. There is no mention of subsequent freezing or of controlling water loss on frozen storage of the seafood.